An MRI apparatus obtains a signal from a test object (a subject being tested) by nuclear magnetic resonance, and produces an image, the subject being placed in a uniform magnetic field. A field of view (FOV) is limited to a uniform magnetic field space generated from a static magnetic field magnet. However, in recent years, an imaging method has been developed, which acquires an image while moving a table with a subject placed thereon, and this enables an imaging of wide field, for example, the whole body of the subject. Imaging the wide field along with such movement of the table as described above needs shortening of imaging time, since a measuring time has to be within a range acceptable by the subject.
As a high-speed imaging method to achieve such reduction of imaging time, there has been developed a technique (referred to as “parallel imaging” or the like, but here referred to as “imaging time shortening technique”), which uses a receiver coil made up of multiple sub-coils and carries out imaging with a wider phase encoding step than a normal phase encoding, whereby aliasing generated in the image are eliminated by utilizing sensitivity distribution information of multiple receiver coils (non-patent document 1). In this imaging method, the number of phase encoding steps can be reduced compared to the normal imaging, and therefore, the imaging time as a whole can be shortened. In theory, thinning-out rate of the phase encoding (=number of phase encoding after thinning-out/number of normal phase encoding) can be assumed as 1/[number of sub-coils], and it is possible to shorten the imaging time in accordance with the thinning-out rate of the phase encoding.
In order to achieve such imaging time shortening technique, first of all, it is necessary that electromagnetic coupling between each of the multiple sub-coils is sufficiently small. If there is an electromagnetic coupling between sub-coils, noises interfere with each other between the sub-coils, and S/N ratio of an image may be deteriorated. Non-patent document 2 discloses a method that utilizes an amplifier with low input impedance for detecting a signal, as a way to suppress the electromagnetic coupling between the two coils. However, this only way is not able to suppress the magnetic coupling completely, if the size of the coil is large relative to a distance between the two coils.
In the imaging time shortening technique, it is required that geometric arrangement of multiple sub-coils is adequately provided. If the geometric arrangement of the multiple sub-coils is not appropriate, the S/N ratio of the image may be partially deteriorated. Specifically, it is desirable that in the geometric arrangement of the multiple sub-coils, a combination of sensitive distributions of the receiver coils covers the imaging area, and these distributions widely differ from one another as possible. As a standard to evaluate the arrangement of the coils, there is a standard called as “g-factor”. This g-factor can be obtained according to the following expression (non-patent document 3).G=√{square root over ( )}{(SHΨ−1S)−1(SHΨ−1S)}≧1  [Expression 1]
In the expression, when the receiver coil has coils, the number of which is indicated by “nc” and the overlapping number of which is indicated by “np”, sensitivity matrix (np×nc) of the overlapping positions is represented by “S”, and the superscript “H” represents a transposed complex conjugate. Ψ represents the noise matrix (nc×nc) of the receiver coil.
The g-factor, which is a value equal to 1 (one) or more, expresses to what extent the pixels being overlapped due to aliasing can be separated, in the coil configuration being utilized.
Accordingly, with regard to the receiver coil used in the imaging time shortening technique, the electromagnetic coupling between the sub-coils and reduction of the g-factor are critical issues.
Conventionally, as for the imaging time shortening technique, a development has been made mainly in the horizontal magnetic field apparatus having a high magnetic field. Various techniques have been proposed for a configuration of the receiver coil that is suitable for the horizontal magnetic field apparatus. In an MRI apparatus, an RF magnetic field in the direction orthogonal to the static magnetic field (z direction) is detected, and generally in the horizontal magnetic field apparatus, the direction of the static magnetic field corresponds to the body axis direction of the subject. Therefore, surface coils 26-1 to 26-10 as shown in FIG. 26(A) to (C) are used as the receiver coils for use in the horizontal magnetic field apparatus. In the surface coils as shown in (A), there are sub-coils in the x-direction and in the y-direction, with different sensitivity distributions, respectively. Therefore, if either the x-direction or y-direction is selected as a phase encoding direction of an MR image, it is possible to remove aliasing of the image. In addition, in the surface coils as shown in (B) and (C), there are sub-coils in three directions x, y, and z, with sensitivity distributions different respectively. Therefore, it is possible to remove the image aliasing, whichever direction is selected as the phase encoding direction.
In addition, as shown in FIG. 27, a combination of various types of coils is proposed as a receiver coil for use in the horizontal magnetic field apparatus (patent document 1). As for this receiver coil, the coils 27-1 and 27-3 are arranged symmetrically with respect to z-axis, thereby avoiding electromagnetic coupling. Moreover, the direction of the magnetic field generated by the coils 27-2 is in the y-direction, and the direction of the magnetic field generated in areas where the coil 27-1 and the coil 27-3 overlap the coils 27-2 is mainly in the x-direction. Therefore, the electromagnetic coupling is weak among them.
On the other hand, as for a vertical magnetic field apparatus, the direction of the static magnetic field is vertical, and in general, the subject is placed so that its body axis is directed to be orthogonal to the static magnetic field. Therefore, a solenoid coil that is arranged around the outer circumference of the subject is used as the receiver coil. The solenoid coil arranged around the outer circumference of the subject has a sensitivity that is intense even in a deep part of the object, unlike the loop coil placed on the surface of the subject. Therefore, if the magnetic field strength is the same, the vertical magnetic field type MRI in which the solenoid coil is available, provides generally a higher sensitivity in the deep part of the object, compared to the horizontal magnetic field type MRI. As the receiver coil for use in the vertical magnetic field apparatus, the patent document 2 discloses, as shown in FIG. 28, a combination of multiple number of solenoid coils 28-1, 28-2, and 28-3 which are arranged around the outer circumference of the subject, and surface coils 29-1 and 29-2. It further discloses that by use of this receiver coil, highly sensitive and high-speed imaging of an area near the heart in the deep part of the subject is performed, by applying the imaging time shortening technique as described in the non-patent document 1.
This receiver coil is effective in imaging a local region such as area in proximity to the heart, however, it is difficult to apply this receiver coil to the wide field imaging, along with movement of the table as described above.
[Non-Patent Document 1]
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